Project summary Retroviruses are enveloped RNA viruses that can infect a variety of species, including humans, and result in chronic infection by integrating the retroviral DNA into the host?s genome. Because of the constant attack of the cellular genome by retroviruses, mammalian cells have developed restriction factors that can prohibit their infection. One of these factors, Serin incorporator 5 (SERINC5) blocks infection of cells by retroviruses by being incorporated in the envelope of the budding virions and preventing the step of the envelope fusion and pore formation with the target cell?s membrane early during infection; thus, rendering the virions incapable of infection. SERINC5 is restrictive to a variety of retroviruses such as human immunodeficiency virus (HIV), murine leukemia virus (MLV) and equine infectious anemia virus (EIAV). Previous work showed that Nef and glycoGag, an HIV and an MLV protein respectively, inhibit SERINC5 by sequestering it from the plasma membrane and thus blocking SERINC5 incorporation inside the budding virions -rendering them fully infectious. While a lot of information is known about the function of SERINC5 in vitro, little is known about its function in vivo. Here we propose to examine the in vivo function of SERINC5 during retrovirus infection, something that has not been hitherto done. We plan to do that by using two approaches: a) by utilizing SERINC5 KO mice and b) using a glycoGag mutant virus or a Nef-expressing- MLV virus. In this work we propose to examine the role of SERINC5 in restricting retrovirus infection in vivo and its role in milk borne transmission of retroviruses. Furthermore, by using a Nef-expressing MLV virus, we intend to examine the interplay of Nef-SERINC5 in vivo and the importance of this interaction during in vivo infection. This study will provide much needed insight into the role of SERINC5 during retrovirus infection in vivo, and also has the potential to create new models for testing therapeutic strategies for treating retroviral infections in humans.